The present invention is concerned with a method to check the liquid take up of a test layer of an analysis element, as well as a corresponding analytical system comprising analysis elements and an evaluation instrument designed to evaluate them.
So-called carrier-bound tests are used on a large scale to analyze the components in a liquid sample qualitatively and quantitatively, particularly in body fluids of humans or animals. In such tests analysis elements are used where reagents are embedded into one or more test layers. An analysis element is brought into contact with the sample to carry out a reaction. The reaction of the sample and reagent results in a change in the analysis element that is characteristic for the analysis and can be evaluated visually or with the aid of an evaluation instrument (usually by reflection photometry).
The evaluation instrument is generally suitable for evaluating a special type of analysis element from a particular manufacturer. Thus, the analysis elements and the evaluation instrument form mutually matched components and are together usually designated as an analytical system.
Numerous different analysis element types are known which vary with respect to the measuring principle (e.g. optical or electro-chemical) and reagents used as well as other design features, in particular the arrangement and fixation of the test layers. Strip-shaped analysis elements (test strips) are particularly common, which comprise a lengthy plastic strip (xe2x80x9cbase stripxe2x80x9d) and at least one test layer affixed to it. Another common analysis element type uses a plastic frame similar to a photographic slide, framing at least one test layer.
Test layers comprise an absorbent material, such as paper or porous plastic. When brought into contact with the liquid sample to carry out an analysis, they absorb liquid. For correct analysis, it is important that the liquid is taken up regularly and completely. In each test the same quantity of liquid corresponding to the absorbency of the test layer is to be taken up and evenly distributed on the test layer.
Visually checking liquid take up by the user is not very reliable. This is particularly true in the case of analysis elements for checking the blood-sugar level used by diabetics, whose sight is frequently impaired by their illness. This is why numerous proposals have already been made to check the liquid taken up by test layers with technical equipment using a measuring method carried out by the evaluation instrument.
For example, in EP-0087466, an analytical system is described in which the quantity of the sample taken up by the test layer is estimated, based on the optical absorption of water in the infrared range. To generate a light signal characteristic of the liquid take up, the test layer is illuminated with primary light (infrared light with a wavelength of some 2 xcexcm), and the secondary light diffusely reflected from the test layer is detected using a light receiver. The intensity of the reflected light is determined when the analysis element is dry as well as after it was contacted with the sample. To obtain information about the quantity of liquid taken up in the test layer, the difference between the two light signals is compared with a reference signal.
Further known methods using an evaluation instrument to check that test layers completely absorb the liquid are described in EP 0473241 A2, U.S. Pat. No. 5,114,350 and DE 19628562 A1. A common feature of these known methods is that the test layer""s surface is illuminated with primary light, and that the intensity of the light diffusely reflected by the test layer is observed by means of a detector directed onto the test layer""s surface while the liquid is being taken up in the layer. The liquid penetrating the test layer causes a reduction of the diffusely reflected intensity. The point in time and degree of change in intensity of the diffusely reflected light is used to obtain information about the point in time and completeness of the liquid taken up in the test layer.
This known measuring principle, however, has considerable disadvantages. In particular, the magnitude of the signal change with liquid take up is very much dependent on the optical absorption of the liquid at the measuring wavelength of the primary light. This is why in the spectral area of visible light an easily measurable, strong signal change only occurs with strongly colored liquids, such as whole blood. If, on the other hand, the liquid taken up into the test layer is only very slightly colored in the visible spectrum (as, for example, with serum or urine), the signal change is very small, and the checking of liquid take up therefore unreliable. It is in particular sensitive to disturbances, such as slight alterations in the position of the test layer caused by unintentional touching of the analysis element in the evaluation instrument.
On this basis, the invention addresses the problem of providing a method and a device for checking the liquid take up of a test layer with improved reliability.
This problem is solved by a method in accordance with claim 1 and an analytical system in accordance with claim 5. Preferred designs are the subject matter of the subclaims.
Numerous different types of analysis elements are known for which the invention can be advantageously used.
In a common type of analysis elements a plurality of test layers are arranged over one another in such a way that their main surfaces are in contact with each other, allowing an exchange of liquid therethrough. The test layers stacked over one another are together designated as the xe2x80x9ctest fieldxe2x80x9d. The sample liquid is usually applied onto the topmost layer of the test field and gradually penetrates the test layers underneath until finally the lowest layer takes up the liquid. Due to the fact that the liquid is basically distributed perpendicular to the test layers"" main surface direction with such analysis elements, they are designated as xe2x80x9canalysis elements with perpendicular liquid transportxe2x80x9d.
In xe2x80x9canalysis elements with longitudinal liquid transportxe2x80x9d, several test layers are arrangedxe2x80x94usually on a base stripxe2x80x94directly next to one another, whereby they are held together in contact on the edge, enabling liquid to pass over from test layer to test layer parallel to their main surface direction.
Finally, combinations of these two principles are known, as, for example, described in U.S. Pat. No. 5,096,836.
As a rule, checking liquid take up refers to the last test layer in the path of liquid flow of the analysis element. In principle, however, the invention is suitable for checking the liquid take up of any absorbent test layer of an analysis element, insofar that this test layer is in direct contact with an optically transparent foil segment. Here, the term xe2x80x9cabsorbentxe2x80x9d is generally to be understood as a designation for any form of liquid take up and not only includes absorbency by porous materials caused by capillary effect, but also liquid take up based on a swelling process.
In the design of analysis elements optically transparent plastic foils are mainly used as test layer carriers, where the test layer is permanently fixed to a foil segment. In particular, the foil segment forming a test layer carrier may be directly coated with the test layer. This is customary for test layers which do not have self-supporting properties.
An example of this type of test layers are so-called test films, i.e. test layers manufactured by coating a substrate with a thin film of a film-forming material. In particular, test films capable of swelling on the basis of gelatine as well as non-swelling porous test films on the basis of a dispersion film former are known. The film-forming mass used to form such a film generally comprises, apart from reagents, solid components, such as pigments and diatomaceous earth that provide the desired opacity and absorbency. The invention is in particular suitable for test layers comprising a dispersion film former and a pigment. Such test layers are described, for example, in U.S. Pat. No. 5,169,787 and 5,536,470.
The transparent foils used in analysis elements (particularly as test layer carrier foil) have a very small thickness of usually less than 0.2 mm. The surface area of the circumferential surface (i.e. of the surface generated by cutting the foil and running vertical to the foil""s main surface direction on its circumference) is accordingly very small. With a square foil segment with 6 mm edge length, for example, the circumferential surface comprises four sections that each have an area of only 1.2 mm2, compared to which the main surfaces (topside and underside) each have a surface area of 36 mm2. Consequently the exit cross-section for light from the circumferential surface is very small, but it has been determined within the scope of the invention that the intensity of light emerging from the circumferential surface is high enough to be measured without difficulty.
A particular advantage is that the percentage change in the intensity of light coming from the circumferential surface during liquid take up is considerably higher than the intensity change of the diffusely reflected light detected in the known methods of checking liquid take up. This is in particular true even in cases, in which the liquid taken up by the test layer has a weak color. A relatively strong signal change is observed even when the test layer takes up distilled water. Therefore the method according to the invention achieves with simple means an increased reliability for checking liquid take up. The method is very robust against disturbances, such as changes in primary light or in the position of the analysis element in the evaluation instrument caused by involuntary contact.
The method according to the invention selectively detects the moistening of the limiting layer between the test layer and the foil segment. Complete liquid take up is therefore more reliably detected than with the known methods, in which the measuring signal refers to the average value of the test layer volume.